Since the above production method of the sintered machine part by powder metallurgy is characterized in that (1) a near-net shape can be formed, (2) it is suitable for mass production, (3) special materials, which cannot be made of ingot material, can be produced, or the like, it can be applied to machine parts for automobiles and machine parts for various industries.
As an example in which especially characteristic (3) of the above characteristics is utilized, a powder magnetic core used for an iron core such as an ignition coil, etc., may be mentioned (Patent Publications 1 to 3, etc.). Generally, the powder magnetic core is produced by forming an insulation coat on the surface of soft magnetic powder such as iron powder, compressing a mixed powder in which a small amount of resin powder is added thereto, and carrying out heat treatment. Such a powder magnetic core has a characteristic in which eddy current loss is small, since eddy current generated arises during use is contained inside of the soft magnetic powder by an insulation coat and resin which covers the surface of the soft magnetic powder, because of a high specific resistance value.
A laminated silicon steel plate is used for the iron core, etc., so far. The laminated silicon steel plate is produced by stacking plural silicon steel plates inserting insulating firms between the silicon steel plates. The silicon steel plate contains silicon for improving electric resistance, and has a isotropic crystal direction which is easily magnetized by arranging to a rolling direction. Therefore, the laminated silicon steel plate has a high specific resistance value and small eddy current loss, and is widely used as an iron core. However, since the silicon steel plate is hard and is poor in formability, a target shape is stacked by laminating silicon steel plates blanked as a piece. Therefore, the laminated silicon steel plate has a problem in that efficiency of productivity is low. With regard to this point, since the powder magnetic core has the above characteristics (1) and (2), it is used instead of the laminated silicon steel plate.
The iron core for an ignition coil is inserted into a primary coil and a secondary coil. It is known that magnetic property is maximally exhibited when there is no gap between these coils and the iron core. Therefore, the most suitable shape of the iron core is a simple cylinder in which a cross section is circular having an outer diameter in proportion to an inner diameter of the coil.
In the case in which the raw material powder is compressed into the simple cylinder, as shown in FIG. 19A, generally a method described below is used. That is, a suitable amount of the raw material powder is filled in a cavity formed by the die hole 21 formed in a die 2 which extends in a longitudinal direction and the lower punch 4 slidably inserted from below into this die hole 21. Here, the cavity is formed an upper portion of a die hole 21 than a lower punch 4. Next, an upper punch 3 slidably inserted into the die hole 21 is inserted from above. And the raw material powder is compressed by the upper punch 3 and the lower punch 4. According to this method, an end surface 10 is formed by the upper punch 3 and the lower punch 4, as shown in FIG. 19B, and the compact 1 in which side surfaces extending to an axial direction are formed by an inner surface of the die hole 21 of the die 2 can be produced. That is, this compressing method is a method which compresses the powder in an axial direction of the compact in order to form the cylindrical compact.
However, when this method is used, a phenomenon in which density of an upper end portion and a lower end portion (both ends in an axial direction) in the compact 1 is higher than the density of a center portion b and the density of the center portion b is decreased, that is, a so-called neutral zone is generated at a center portion in a compressing direction. The neutral zone is generated due to the following reason. The powder compressed by the upper punch 3 and the lower punch 4 is rearranged by sliding at a powder surface due to pressure, and is refined by plastically deforming. However, pressure applied by the upper punch 3 and the lower punch 4 is transmitted from an end portion a to a center portion b, while being consumed by rearrangement and plastic deformation. Therefore, pressure applied to the raw material powder decays, as distance between the compact 1 and the end surface 10 is increased.
In the case in which length of the compact in a compressing direction to a compressed surface is long, generation of the neutral zone is not avoided, and it is difficult to eliminate the neutral zone, even if the pressure applied by the upper punch and the lower punch is increased. In the above powder magnetic core for an iron core, when a neutral zone having low density is generated, magnetic properties are deteriorated depending on the generation, since in particular, magnetic flux density is proportional to space factor of the soft magnetic powder.
Therefore, when a substantially cylindrical compact which is long in an axial direction is compressed, a method in which powder is compressed in a radial direction crossing an axial direction of the compact at right angles, is proposed (Patent Publications 4 and 5, etc.). The publications describe that according to this method, distance in a compressing direction is short, and an entire side surface is pressed by the upper punch and the lower punch. Therefore, a compact in which the neutral zone is not generated, density is uniform in a longitudinal direction and is high, can be produced.
Patent Publication 1 is Japanese Unexamined Patent Application Publication No. H03-238805.
Patent Publication 2 is Japanese Unexamined Patent Application Publication No. 2006-278499 (claim 8).
Patent Publication 3 is Japanese Unexamined Patent Application Publication No. 2008453611.
Patent Publication 4 is Japanese Patent Publication No. H03-013281.
Patent Publication 5 is Japanese Examined Patent Application Publication No. 2005-240060.